freedreno: a2xx: Change use of BLEND_ to BLEND2_

freedreno: a2xx: Change use of BLEND_ to BLEND2_

Change use of BLEND_ to BLEND2_,

    BLEND_* a3xx_rb_blend_opcode
    BLEND2_* is a2xx_rb_blend_opcode

This makes no effective difference as the used enumerant has the same
value (0), but the other enumerants do not match 1-to-1 so this will
avoid future problems.

Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Rob Clark <robdclark@gmail.com>